At the time of the occurrence, when the bow section of the jet boat SauteMoutons6 was immersed in the fourth large wave at Devil's Hole, the rate of upward acceleration of the bow was directly related to the speed of the vessel. The acceleration was further increased by the upward velocity of the water within the wave, which was caused by a current of over 20knots. Because of the short distance between the wave crests, the stern section remained high due to the lift from the previous crest, and the vessel experienced a pivoting action. All the forces acting on the hull section forward, in conjunction with the pivoting action, combined to make the bow snap upward rather than move smoothly and symmetrically upward upon hitting the next wave. In addition, there was a rapid deceleration of the vessel. This caused the passengers on the starboard side forward, who appeared to be more subject to the accelerations and decelerations, to move rapidly ahead and up. As no other passenger suffered similar injury, it can be deduced that either the accelerations experienced in this part of the boat were greater than in other areas, or the force the three injured passengers exerted against the bar to stabilize themselves against the accelerations was insufficient to remain immobile in their seats. The tour videotape appeared to show that one of the injured passengers, had attempted to stand. However, as all three passengers suffered similar injuries, it is more likely that she was propelled to an upward position during the injury sequence. Given that this passenger was taller than the others around her, the perception that she was standing was most likely due to the aft camera angle. The injuries received by the passengers when they fell back into the padded seating may have been due in some measure to the degree of protection offered by the cushioning. Because video records of this and other trips were scrutinised and there appeared to be little difference in wave height or direction or in the vessel's approach, the variable which may have caused the vessel to react in the way that it did was the vessel's speed. Other factors in the occurrence were the handling characteristics of the boat and the ability of the operator. Engine power is instantly translated into vessel movement - movement of the controls results in an instantaneous response. Over-application of power or steering can result in a sudden, unwanted movement of the vessel. Decisions by the operator as to throttle setting and direction must be resolved in fractions of a second while negotiating the whitewater. Had the circumstances permitted the speed of the vessel to be lower, the resulting snap action forward, which propelled the passengers from their seats, would most likely have been less violent. The scheduled tour was on time; there was no indication that the vessel had been proceeding at a higher-than-normal speed over the whole trip. The raising of arms at intermittent times during the trip is used to signal the well-being of passengers, but this system may have serious drawbacks. Because the vessel continues to pitch and gyrate over the waves, the time intervals between raising arms may be too great for an injured passenger to promptly signal that something is wrong. Immediately after the event that injured the passengers, the SauteMoutons6 continued to run through the whitewater at the same speed. It was only when the passengers were requested to raise arms a short time later, that the operator knew that all was not well.Analysis At the time of the occurrence, when the bow section of the jet boat SauteMoutons6 was immersed in the fourth large wave at Devil's Hole, the rate of upward acceleration of the bow was directly related to the speed of the vessel. The acceleration was further increased by the upward velocity of the water within the wave, which was caused by a current of over 20knots. Because of the short distance between the wave crests, the stern section remained high due to the lift from the previous crest, and the vessel experienced a pivoting action. All the forces acting on the hull section forward, in conjunction with the pivoting action, combined to make the bow snap upward rather than move smoothly and symmetrically upward upon hitting the next wave. In addition, there was a rapid deceleration of the vessel. This caused the passengers on the starboard side forward, who appeared to be more subject to the accelerations and decelerations, to move rapidly ahead and up. As no other passenger suffered similar injury, it can be deduced that either the accelerations experienced in this part of the boat were greater than in other areas, or the force the three injured passengers exerted against the bar to stabilize themselves against the accelerations was insufficient to remain immobile in their seats. The tour videotape appeared to show that one of the injured passengers, had attempted to stand. However, as all three passengers suffered similar injuries, it is more likely that she was propelled to an upward position during the injury sequence. Given that this passenger was taller than the others around her, the perception that she was standing was most likely due to the aft camera angle. The injuries received by the passengers when they fell back into the padded seating may have been due in some measure to the degree of protection offered by the cushioning. Because video records of this and other trips were scrutinised and there appeared to be little difference in wave height or direction or in the vessel's approach, the variable which may have caused the vessel to react in the way that it did was the vessel's speed. Other factors in the occurrence were the handling characteristics of the boat and the ability of the operator. Engine power is instantly translated into vessel movement - movement of the controls results in an instantaneous response. Over-application of power or steering can result in a sudden, unwanted movement of the vessel. Decisions by the operator as to throttle setting and direction must be resolved in fractions of a second while negotiating the whitewater. Had the circumstances permitted the speed of the vessel to be lower, the resulting snap action forward, which propelled the passengers from their seats, would most likely have been less violent. The scheduled tour was on time; there was no indication that the vessel had been proceeding at a higher-than-normal speed over the whole trip. The raising of arms at intermittent times during the trip is used to signal the well-being of passengers, but this system may have serious drawbacks. Because the vessel continues to pitch and gyrate over the waves, the time intervals between raising arms may be too great for an injured passenger to promptly signal that something is wrong. Immediately after the event that injured the passengers, the SauteMoutons6 continued to run through the whitewater at the same speed. It was only when the passengers were requested to raise arms a short time later, that the operator knew that all was not well. Except for momentary requirements for increased rpm; which may have suddenly increased the speed at the time of the occurrence, the engines of the SauteMoutons6 were, over the time period allotted to complete the trip, operated at a normal speed. Three passengers in the forward section of the seating area were injured when they were propelled out of their seats by the movement of the SauteMoutons6. The injured passengers did not, or could not, stabilize themselves with sufficient force to prevent being propelled out of their seats. The injured passengers were not able to instantly signal their distress to the operator or crew. There was a lapse of time between when the passengers sustained their injuries and when they were identified by the crew as having been injured. During this time the vessel continued to pitch and gyrate over the whitewater. The foam within the seat bench was not capable of sufficiently cushioning the impact sustained by each of the injured passengers.Findings Except for momentary requirements for increased rpm; which may have suddenly increased the speed at the time of the occurrence, the engines of the SauteMoutons6 were, over the time period allotted to complete the trip, operated at a normal speed. Three passengers in the forward section of the seating area were injured when they were propelled out of their seats by the movement of the SauteMoutons6. The injured passengers did not, or could not, stabilize themselves with sufficient force to prevent being propelled out of their seats. The injured passengers were not able to instantly signal their distress to the operator or crew. There was a lapse of time between when the passengers sustained their injuries and when they were identified by the crew as having been injured. During this time the vessel continued to pitch and gyrate over the whitewater. The foam within the seat bench was not capable of sufficiently cushioning the impact sustained by each of the injured passengers. The rapid accelerations and decelerations experienced by the vessel and passengers caused the pelvic regions of the three passengers to forcefully strike the seat bench of the vessel, causing vertebrae and back injuries. The possibility that these passengers may not have braced themselves in their seats as instructed may have contributed to their injuries. The cushioning ability of the seat appears to have been inadequate for the movements of the SauteMoutons6 at the time of the occurrence and was a contributing factor.Causes and Contributing Factors The rapid accelerations and decelerations experienced by the vessel and passengers caused the pelvic regions of the three passengers to forcefully strike the seat bench of the vessel, causing vertebrae and back injuries. The possibility that these passengers may not have braced themselves in their seats as instructed may have contributed to their injuries. The cushioning ability of the seat appears to have been inadequate for the movements of the SauteMoutons6 at the time of the occurrence and was a contributing factor. At the time of the occurrence, the vessel's owners were studying various ways of improving the cushioning of the passenger seats and the degree to which injuries could be mitigated by improvements in seat design and cushioning. This study includes the possibility of using liquid filled satchels in the shape of cushions. Since the occurrence a more substantial foam padding has since been fitted to the passenger seats.Safety Action At the time of the occurrence, the vessel's owners were studying various ways of improving the cushioning of the passenger seats and the degree to which injuries could be mitigated by improvements in seat design and cushioning. This study includes the possibility of using liquid filled satchels in the shape of cushions. Since the occurrence a more substantial foam padding has since been fitted to the passenger seats.